1. Field of the Invention
The present invention relates to a controller for a gas concentration sensor, and more particularly to a gas concentration sensor controller that is suitable for the control of a gas concentration sensor installed in an internal-combustion engine's exhaust path.
2. Background Art
As disclosed by Japanese Patent JP-A No. 28575/2000, there is a conventionally known device that includes an oxygen sensor installed in an internal-combustion engine's exhaust path. The oxygen sensor for this device generates an output in accordance with the oxygen concentration in an exhaust gas that flows in the exhaust path. There is a correlation between the oxygen concentration in the exhaust gas and the exhaust air-fuel ratio. With the conventional device, it is therefore possible to obtain the information about the exhaust air-fuel ratio in accordance with the oxygen sensor output.
The above device is capable of detecting the device impedance of the oxygen sensor by varying the voltage V0, which is applied to the oxygen sensor, from a reference voltage to a sweep voltage. If a ΔV0 change occurs in the applied voltage V0, the associated current I changes by ΔI, which corresponds to the device impedance Rs. Therefore, the above conventional device calculates the device impedance Rs in accordance with the voltage change ΔV0 and current change ΔI, which arise when the applied voltage V0 changes from the reference voltage to the sweep voltage.
As described above, the above conventional device acquires the information about the exhaust air-fuel ratio in accordance with the oxygen sensor's output, and detects the device impedance by applying the sweep voltage to the oxygen sensor. While the sweep voltage is applied to the oxygen sensor, the output value of the oxygen sensor is affected by the sweep voltage. Therefore, while the sweep voltage is applied, the oxygen sensor's output does not correspond to the exhaust air-fuel ratio.
The oxygen sensor includes an impedance component and a capacitance component. Therefore, the oxygen sensor's output does not revert to a value corresponding to the exhaust air-fuel ratio for some time after the application of the sweep voltage is stopped. Consequently, the above conventional device may erroneously detect the exhaust air-fuel ratio during the time interval between the instant at which the sweep voltage is applied to the oxygen sensor for device impedance detection purposes and the instant at which the influence of the sweep voltage disappears.